Process of nitrating cellulose



Jan. 18, 1938. L. s. BAKER ET AL PROCESS OF NITRATING CELLULOSE Filed Sept. 25, 1936 2 Sheets-Sheet l Ivan H /W0zgcm Ra/p/2 J Quad m L P M F. T 5m R N 0 3 E INVNTORS BY J ' M C17 ATTONEY.

Jan. 18, 1938.

FILTRATION YIELD L. s. BAKER ET AL PROCESS OF NITRATING CELLULOSE Filed sept. 25, 1936 2 Sheet-Sheet 2 uso n70 n90 1210 1250 N|TROGEN CONTENT H N05 OO G RAPH -]Y ACID COM POSITION CHART NITR|C ACID CONTENT BETWEEN |5'7a AND 452 FOR l27. NITROGEN NITROCELLULOSE PRIOR TO VISCOSITY REDUCT|ON CONSTANT NITROGEN LINE |27a OF MAX|MUM METHL ALCOHOL COMPATIBILITY Low2s J.Ba/cer INVENTORS A'ITO EY.

Patented Jan. 18, 1938 UNITED STATES PATENT OFFICE River, and

Ralph J. Quaid,

Sayreville,

N. J., assiglnors to E. I. du Pont de Nemours & Company. Wilmington, Del., a corporation of Delaware Application September 25, 1936, Serial No. 102,512

6Claims.

This invention relates to a process of preparing cellulose nitrate and more particularly to a process for preparing cellulose nitrate which is soluble in methyl alcohol. During the war period when cellulose nitrate was manufactured in previously unthought of quantities it was the natural course of development to reduce to the lowest possible value the nitric acid content of the nitrating mixture as this was the most expensive ingredient. It was found that cellulose nitrate, such as was used in smokeless powder manufacture, could be prepared using a nitrating acid containing approximately 20% and in some cases even less nitric acid. With the growth of a soluble cellulose nitrate industry subsequent to the war, there was the natural adoption of this economical nitrating mixture. While the solubility characteristics of the cellulose nitrate resulting from the use of a nitrating mixture containing approximately20% nitric acid content permitted its use to a certain degree, it was somewhat limited due to a lack of solubility at c'oncentrations that would allow economicai operation. This lack of solubility is nota viscosity efiect as at the standard viscosity hereinafter mentioned it was possible to prepare usable methyl alcohol solutions containing only 10 to 15% cellulose nitrate and these would usually gelatinize after a few days duration.

However, the inventors have found that by increasing the nitric acid content of the nitrating mixture to 30% that it is possible at the same viscosity as above to prepare methyl alcohol solutions containing 25% cellulose nitrate that exhibit no gelatinization tendencies. Furthermore,

the inventors have io11d that despite the marked increase in the nitric acid content of the nitrating mixture the consumption of this expensive ingredient during the manufacture has not been increased.

An object of this invention is the provision of aprocess for nitrating cotton linters which results in a cellulose nitrate which is completely soluble in methyl alcohol. Another object of this invention is the provision of a method of nitrating cotton linters which results in a product which can be filtered with case if necessary. A further object of this invention is the provision of a process for nitrating cellulose which yieids cellulose nitrate the solutions of which are exceptionally clear whether dissolved in methyl alcohol or conventional solvents. A still further object is the provision of a method in which cotton linters are very uniformly nitrated.

The above objects are accomplished according to the present invention by controlling the percentage of nitric acid in the nitrating mixture, the water content of the nitrating mixture, the temperature at which the nitration, is carried out. and the ratio of linters to acid.

In carrying out the process of this invention, the objects mentioned above are obtained if the percentage of nitric acid in the nitrating mixture is adjusted to about 30%. The most satisfactory nitric acid rangehas been found to be from 27% to 32%. If nitrating acids of a lower or higher percentage of nitric acid are used, the solubility of the resulting product in methyl a1- cohol is greatly reduced.

Since it has been found that the range of optimum methyl alcohol solubilities is effected between the nitrogen content limit of 11.85% to 12.00%, the water content of the nitrating acid should be so controlled as to give a product with a final nitrgen of about 11.92% with 11.85% to 12.00% being the lmiting values.

The temperature at which the nitration is carried out is also critical. The optimum temperature for obtaining cellulose nitrate of the best solubility is about 45 C. Some variation from this temperature may be tolerated On the low side of this range, but more than 1 or 2 degrees above 45 C., is detrimental to the process. If the temperature should be increased to 50 C., gelatinization of the cellulose in the acid will occur and the product will be extremely difiicult to filter.

The critical nature of these and other variables are illustrated in the attached drawngs in which Figure 1 is a graph in which the arbitrary filtration yield described later is plotted against nitrating temperature. Figure 2 represents a graph in which the filtration yield is plotted against the nitric acid content of the nitrating acid mixture. Figure 3 represents a graph in which the flltration yield is plotted against the nitrogen content of the nitrocellulose produced according to the present Invention. Figure 4 represents a triangular graph showing the relation between the nitric acid content, percentage of nitrogen in the nitrocellulose prior to viscosity reduction, sulphuric acid water content, and the arca in which the maximum compatibility of the product and methyl alcohol is obtained.

In carrylng out the invention, it is preferred to treat each part of cotton linters with about 50 parts of nitrating acid. This limitation, however, is not as critical as those mentioned above.

It is also preferred to allow the acid to react with the cellulose tor about 20 minutes.

Example 1 Purified cotton linters containing about 2% or less moisture are immersed in approximately 50 times their weight of nitrating acid mixture having the following composition:

Percent Nitric acid 30.7 Sulphuric acid 51.7 Water 17.6

The bath is agitated for 20 minutes at a temperature of about 45 C. The reaction is carried out in a mechanical clipping pot of the type well known in the industry. At the completion of the nitration, the excess acid is removed by centrifuging and the cellulose nitrate is stabllized by boiling in acidulated water and then washing until acid-free in the usual manner. The cellulose nitrate so prepared usually has a nitrogen content of about 11.98%. The viscosity of the product at this stage is about 500 to 1000 seconds. In order to secure maximum methyl alcohol compatibility characteristics, the viscosity of the product is reduced to approximately 6 to 15 seconds. This may be done by the pressure reduction process well known in the art or by the ammonia reduction method described in U. S. Patent No. 2,039,381 issued to Bacon. The viscosity of the cellulose nitrate is determined according to A. S. T. M. specifications, D30133, formula A. The reduction in viscosity usually reduces the percentage of nitrogen approximately .06%. This usually gives a value of 11.90 to 11.95 as the percentage of nitrogen in the cellulose nitrate. The product obtained according to this invention is practically completely soluble in methyl alcohol.

The degree of solubility is determined by a filtration yield method. This is carried out by preparing a dispersion of 24 parts of cellulose nitrate in 72.5 parts of methyl alcohol and 3.5 parts of camphor. This solution is filtered through filter paper under a pressure of about 50 lbs. per sq. in. and the result is given in grams of exuded bone dry cellulose nitrate per sq. centimeter of filter paper area. The apparatus in which this test is carried out consists of a filtration tube Which is made from a brass cylinder 10 inches long having an inside diameter of 1% inches.- It is provided with threaded brass caps and operates in a vertical position. The bottom cap is perforated and holds a, round piece of felt approximately an of an inch thick. On top of the felt is placed a Carl Schlelcher 520B filter paper, and the felt and paper are held in place by a metal ring of a known inside arca. This assembly is then placed over the bottom end of the tube and screwed up tightly. The tube is then filled with the solution to be tested and air pressure Which is held at 50 lbs. per sq. in. by an automatic regulator is supplied to the space above the solution. This pressure is applied until the flow of material through the filter stops. The amount of filtrate per sq. centimeter of filtering area is then determined. A cellulose nitrate which yields a filtrate containing more than 5 grams of bone dry cellulose nitrate per sq. centimeter in such an apparatus is considered very satisfactory.

Cellulose nitrates prepared according to the present invention are completely soluble in methyl alcohol where the nitrogen range lies between 11.85% and 12%. In this range, 11.92% gives the optimum results as illustrated in the example. The cellulose nitrate prepared according to this invention in addition to being completely soluble in methyl alcohol oiers the advantage that it is exceptionally free from un-nitrated fibers Which results in a solution of exceptional clarity, freedom from haze, and brilliance. Furthermore, the improved product derived from this invention is soluble at concentrations of 25% cellulose nitrate in methyl alcohol and even greater concentrations, which is conducive to very economical operation as compared to products previously available. The invention also presents the advantage that the films produced from methyl alcohol solutions are exceptionally strong and may be prepared from methyl alcohol which is relatively cheap. Another advantage from an economical standpoint is that the filtration step of conventional processes may be eliminated.

It is apparent that many widely different embodiments of this invention may be made without departing from the spirit and scope thereof; and, therefore, it is not intended to be limited except as indicated in the appended claims.

We claim:

1. Process of preparing methyl alcohol soluble cellulose nitrate which comprises nitrating cellulose with nitrating acid containing about 30% nitric acid at a temperature of about 45 C. for twenty minutes, said cellulose nitrate having a nitrogen content between approximately 11.85 and 12.0%.

2. Process ofpreparing methyl alcohol soluble cellulose nitrate Which comprises treating one part of cotton linters with about fifty parts of nitrating acid containing 27 to 32% nitric acid at a temperature of approximately 45 C. for about twenty minutes, said cellulose nitrate having a nitrogen content between approximately 11.85 and 12.0%.

3. Process of preparing methyl alcohol soluble cellulose nitrate which comprises nitrating about one part of cotton linters with about fifty parts of nitrating acid containing about 30% nitric acid, at a temperature of about 45 C., to a nitrogen content of about 12%, reducing the viscosity thereof to 6 to 15 seconds, and dissolving the product in methyl alcohol.

4. Process of preparing methyl alcohol soluble cellulose nitrate which comprises nitrating one part of cotton linters with about fifty parts of nitrating acid having the following composition:

- Percent Nitric acid 30.7 Si1lfiuric acid 51.7 Water 17.6 

